Apicomplexan parasites include a number of pathogens of medical and veterinary importance and our long- term goal is to identify novel molecules and pathways in these microorganisms that could be targets for chemotherapy, diagnostic applications, or vaccines. Many of these parasites rely for their survival on biosynthetic pathways that occur in a remnant plastid known as the apicoplast. A large number of these anabolic pathways are essential for the life of the parasite. Our laboratory recently found a two-pore channel (TPC) that localizes to the apicoplast membrane(s) and is essential for growth of Toxoplasma gondii. Two-pore channels belong to the superfamily of voltage-gated ion channels and are characterized by the presence of two sets of six-transmembrane domains (TMDs). This was an important discovery because it exposed a completely untapped question, which is the role of ions like Ca2+ on the activity of essential anabolic enzymes of the organelle and how this impacts parasite fitness. In addition, and more importantly, ion channels are targets of many therapeutically useful agents and they remain significantly under-exploited as therapeutic targets, even more so as antiparasitic agents. The apicoplast, the product of a secondary endosymbiosis process, is an organelle surrounded by membranes derived from the cyanobacterium that gave origin to the plastid, the endosymbiotic alga, and the host endosomal compartment. Metal ions play essential roles for the activity of more than one third of all enzymes and considering the large number of essential activities present in the apicoplast, it is likely that ions will be highly regulated in the organelle. No ion transporters or channels have been reported in the apicoplast until now. This makes our recent discovery of a two-pore channel localized to the apicoplast a unique finding and also an unexpected opportunity for the development of new therapies. TPCs are found in both animal and plant cells where they localize to acidic organelles such as endosomes, lysosomes and vacuoles. TPCs function as Ca2+ channels in mammalian cells and are activated by the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). The Toxoplasma TPC localization to the plastid is unique to Apicomplexa and highlights a novel and potentially specific role for the organelle multiple functions. We silenced the T. gondii TgTPC gene and found that the mutants display a severe growth defect that can be rescued by complementation with a functional channel. Our hypothesis is that the TPC is a non-selective channel that upon voltage gating is able to conduct cations but in the environment of the cell it probably prefers Ca2+. This hypothesis implies that the apicoplast contains Ca2+, an idea supported by preliminary evidence. We propose a model in which the apicoplast through the TPC and Ca2+ communicates with other organelles via other potential Ca2+ channels. Two other candidate Ca2+ channels will be studied. Our goal is to define the physiological function of TgTPC using sophisticated biophysical approaches in the context of several model systems. We will combine this with live cell studies to determine how their activity influences the physiology of the apicoplast and other intracellular organelles and the fitness of the parasite.